Non-aqueous secondary batteries (hereinafter, also abbreviated as “secondary batteries”) such as lithium ion secondary batteries have characteristics such as compact size, light weight, high energy density, and rechargeability, and are used in a wide variety of applications. A secondary battery normally includes battery components such as a positive electrode, a negative electrode, and a separator that separates the positive electrode and the negative electrode, and prevents short-circuiting between the positive electrode and the negative electrode. In some secondary batteries, porous membranes are provided on these battery components as protective layers with an objective such as improving heat resistance and strength.
A porous membrane such as described above can for example be formed by non-conductive particles, such as organic particles or inorganic particles, bound by a binder. The porous membrane is typically formed by preparing a slurry composition (hereinafter, also referred to as “slurry composition for a porous membrane”) containing porous membrane materials, such as the non-conductive particles and the binder, that are dissolved or dispersed in a dispersion medium, such as water, applying the slurry composition for a porous membrane onto a substrate, such as an electrode or a separator, and drying the applied slurry composition for a porous membrane.
In recent years, there has been a large amount of activity directed toward improving porous membranes and in particular binders used in porous membranes, with an objective of providing secondary batteries having even higher performance. For example, WO 2012/057324 A1 (PTL 1) proposes a technique of forming a porous membrane using a binder wherein the binder is composed of a polymer containing, in the same molecule, a nitrile group, a hydrophilic group, and a straight chain alkylene structural unit having a carbon number of 4 or more, the content proportion of the nitrile group in the polymer constituting the binder is 1 mass % to 25 mass %, and the iodine value of the polymer is 0 mg/100 mg or more and 30 mg/100 mg or less. The porous membrane in PTL 1 is reported to have excellent heat resistance, flexibility, and adhesiveness and contribute to improved cycle characteristics of a battery.